Micro-Electro-Mechanical Systems, in short MEMS can be defined as miniaturized mechanical and electro-mechanical systems where at least some elements have a mechanical functionality. Since MEMS devices are created with the same or similar tools used to create integrated circuits, micromachines and microelectronics can be fabricated on the same piece of silicon.
MEMS structures can be applied to quickly and accurately detect very small changes in physical properties. For example, a microelectronic gyroscope can be applied to quickly and accurately detect very small angular displacements.
Detection of movement of the movable parts of a MEMS structure may be for example capacitive or piezoelectric. In either case, electrical signals obtained from a moving MEMS structure comprise relatively weak analog signals, and any electrical or magnetic interference may cause errors in these sensitive analog signals, thus deteriorating the performance of the MEMS device. An example of such weak and sensitive analog signals is a sense signal corresponding to movement of at least one part of the MEMS structure.
A mixed-signal multi-chip package refers to a single package comprising at least two integrated circuit (IC) dies, also referred to as chips, wherein both analog and digital signals are processed by the IC dies within the same package. An exemplary mixed-signal multi-chip package may comprise an analog IC die and a digital IC die or a mixed-signal IC die and a digital IC die.
The sensitive analog signals from a MEMS structure are preferably digitized as close to the point of generation as practically possible. One solution is to dispose a MEMS chip comprising the MEMS structure and some front-end analog circuitry into an IC component package body together with an integrated circuit (IC) capable of digitizing analog signals and further processing the digitized signals. This way the distance required for coupling the analog signals for further processing may be minimized. The IC may be for example an application specific integrated circuit (ASIC). However, integration of analog and digital IC's in a single mixed-signal multi-chip package may also introduce problems through coexistence of both sensitive analog signals and relatively strong digital signals. Crosstalk between strong digital signals and sensitive analog signals is one of these problems.
FIG. 1 illustrates a conventional mixed-signal multi-chip component. Sensitive analog signals are carried between the MEMS die (100) and a digital IC die (200) over first bonding wires (101). The first bonding wires (101) are coupled to the dies (100, 200) at first chip pads (113). Digital signals are communicated from the digital IC die (200) towards outside circuitry via leads of a lead-frame, which is marked in the FIG. 1 with diagonal striped fill. At least one digital signal is coupled from a second chip pad (205) to a bond pad of the lead-frame by second bonding wires (211). The bond pad is electrically connected to a lead configured to communicate with circuitry external of the mixed-signal multi-chip component using the at least one digital signal. We may call such bond pad carrying a digital signal as a signal-bearing bond pad (202). Crosstalk may easily occur between a signal-bearing bond pad (202) and a first bonding wire (101) within the volume of the component body, since there is no electromagnetic interference (EMI) protection between the two. If crosstalk causes erroneous values in the sensitive analog signals, the digital processing may not be able correct the erroneous results but assumes the voltage or current caused by crosstalk as being originally provided by the MEMS die (100). Thus, the digital signal does not represent properly the wanted signal comprising information received from the MEMS die (100), but it may be a sum of the wanted analog signal and crosstalk error. Thus, crosstalk reduces accuracy and reliability of detection results achieved with the MEMS device, which detection results are obtained by analysing the sensitive analog signals provided by the MEMS die (100) and carried by the first bonding wires (101). A sensitive analog signal carried by one of the first bonding wires (101) may be referred to as a victim or as a victim signal, whereas the disturbing digital signal carried through the second bonding wire (211) may be referred to as an aggressor and the signal-bearing bond pad (202) may also be referred to as an aggressor bond pad. Likewise, the respective bonding wires may be referred to as a victim bonding wire (101) and an aggressor bonding wire (211).
FIG. 2 is a capture from a simulation that further illustrates the problem in the prior art. Electric potential caused by an aggressor bond pad (202) is illustrated with shades of grey. The whiter the area, the stronger is the voltage caused by the aggressor. In the black area the electric potential caused by the aggressor is not significant. While the victim bonding wires (101) carrying sensitive analog detection signals are all within the area at which the electric potential caused by the aggressor is high, risk of errors caused by crosstalk is significant. In this simulation, an exemplary test voltage of 1V was used in the aggressor bond pad (202). The resulting voltage caused by the aggressor bond pad at the area of the closest victim bonding wires (101) was found to be in the level of 5 mV. As understood by a skilled person, the actual amount of crosstalk and thus the error caused by the crosstalk at the victim bonding wires (101) depends on the nature and level of the aggressor and victim signals as well as various structural aspects of the design, but the simulation provides a good basis level for comparison.